The present invention is with respect to an inocculant for processing cast iron and a process for use of the inocculant.
By definition the amount of carbon in cast iron is greater than 1.7%, small amounts of manganese, phosphorus, sulfur and silicon being present as well. When melted cast iron becomes solid, a part of the carbon is segregated out as graphite (in the cast of gray cast iron) or as Fe.sub.3 C (cementite, in the case of white cast iron). The sort and amount of the impurities and other substances in the iron together with the selection of the rate of cooling, as dependent on the thickness of the casting on hand, have different effects on the solidification of the casting. Because white-solidified cast iron may not be readily machined because of its hardness and brittleness, white solidification is undesired for most uses and is stopped or limited by inocculation of the melted iron.
The inocculant which has been most widely and longest used is ferrosilicon with about 75% of silicon. Its effect is produced not only by the graphitizing caused by its silicon, but, to a great degree, certain controlled amounts of aluminum and calcium. The effect may be further increased by other materials such as barium, zirconium and strontium.
Other normally used inocculants are 30/60 calcium-silicon, mixed ferrosilicon and calciumsilicon and, furthermore, graphite.
It may be generally said that the inocculation of liquid metals is the placing of impurity nuclei in the melted material, which take the form of crystallization centers for the forming of graphite. In this respect, the inocculation effect is said to be caused only by solid crystalization nuclei. For this purpose substances with metallic, and furthermore substances with non-metallic properties, as for example oxides, sulfides, nitrides, borides and carbides may be used, while compounds which are broken down under the effect of heat, go into solution in the melted material or undergo reduction or decomposition may not be used as inocculants.
Generally speaking, there are two forms of inocculation: first-stage inocculation in the ladle as a single-stage process and first-stage inocculation in the ladle together with later, second-stage inocculation right before teeming or solidfying of the melt in the mold.
The overall amount of inocculant used is between about 0.1 and 0.8% by weight. With such amounts white-solidification of the cast iron may be stopped or limited to the desired degree, nucleation of the melt may be made better and graphite crystallization helped. However, such conditions are not without a parallel effect on the properties of the completed casting: With an increase in the amount of inocculant, undesired properties are produced, such as a decrease in hardness, an increase in blowholes, porosity, greater cracking, thicker flaking of graphite, amongst others.